Cyanide ion (CN<sup>-</sup>) has been the subject of numerous studies probing the mechanisms underlying the surface-enhanced Raman scattering (SERS) phenomenon. This work examines various aspects critical to application of SERS for direct detection of trace cyanide in groundwater and in wastewater streams. A new method for direct quantitation of cyanide in aqueous electrolyte has been developed that uses an <i>ex situ</i> oxidation-reduction cycle to precondition a planar silver electrode. Cyanide ion can be detected in 0.1 M KCI with a linear response between 100 ppm and 10 ppb. The estimated limit of detection is approximately 8 ppb. The effects of pH, electrolyte level, and two common background ions, nitrate (NO<sub>3</sub>-) and sulfate (SO<sub>4</sub><sup>2-</sup>), have been characterized. Cyanide response was found to be sensitive to pH, with optimal performance observed at neutral to basic pH. Electrolyte concentrations of 0.001 M reduced response to cyanide by a factor of five, while levels above 0.1 M had no significant effect. The addition of 10-ppm sulfate ion decreased response approximately 40%, while the presence of nitrate ion at concentrations up to 100 ppm had a negligible effect on SERS response. These results suggest that cyanide ion can be detected directly in high-ionic-strength aqueous solutions, such as groundwater.
Robert D. Shelton, John W. Haas, and Eric A. Wachter, "Surface-Enhanced Raman Detection of Aqueous Cyanide," Appl. Spectrosc. 48, 1007-1010 (1994)